Lubricant Additives

Organic Friction Modifi ers 203

The mechanisms of operation of this class of products are not fully understood, but the follow-
ing hypotheses are presented:

Diffusion of molybdenum into the asperities

Formation of polymer-type fi lms

In situ formation of molybdenum disulfi de (most accepted hypothesis)

Selective transfer of metal (copper) leading to the formation of thin, easy-to-shear metal fi lms

7.5.2 MECHANICAL TYPES

In this group, the classical types such as graphite and molybdenum disulfi de as well as some more
recent FMs such as tefl on (polytetrafl uoroethylene, PTFE), polyamides, fl uoridized graphite, and
borates can be found. The friction-reducing mechanisms can be explained by

The stratifi ed structure and formation of easy-to-shear layers

The formation of elastic or plastic layers on the metal surface

7.6 FACTORS INFLUENCING FRICTION-REDUCTION PROPERTIES

This section lists the main factors that impact friction-reducing properties.

1. Competing additives. Other polar additives with affi nity to metal surfaces such as AW/EP
   and anticorrosion additives as well as detergents and dispersants may compete with FMs.
   This emphasizes that lubricant formulations have to be balanced carefully to achieve opti-
   mal performance.


2. Contaminants. Short-chain acids, which are formed by oxidative degradation of the
   lubricants, may compete at the metal surface, resulting in a loss of friction-modifying
   properties.


3. Metallurgy. The type of steel alloy used will affect the adsorption of FMs.


4. Concentration. Increase of FM concentration results in an increase of friction reduction up
   to a point above which improvements are marginal. Generally, the friction-reducing effect
   is most (cost-)effective at concentrations of ∼0.25 to 1% for organic FMs and 0.05–0.07%
   for molybdenum dithiocarbamates.

7.7 FRICTION MODIFIERS: CURRENT PRACTICE

The most frequently used organic FMs include

1. Long-chain fatty amides, specifi cally oleylamide (Figure 7.6). This is a reaction product
   of olein (main component oleic acid, a straight-chain unsaturated C18 carboxylic acid) and
   ammonia (NH 3 ).

• • • • • •

TABLE 7.5

Classifi cation of Other FMs

Types of FMs Products

Metallo-organic compounds Molybdenum and copper compounds

Mechanical types Molybdenum disulfi de, graphite, tefl on (PTFE)